1. Field of the Invention
The present invention relates to image capturing apparatuses and methods, computer programs, and computer-readable recording media therefor, and more particularly relates to an image capturing apparatus and method, a computer program, and a computer-readable recording medium therefor for use in capturing an image of a subject and generating or reproducing image data from the captured image.
2. Description of the Related Art
A color-space selecting method for use in known image capturing apparatuses will now be described using an example of a digital camera.
In known digital cameras, an sRGB color space is generally used to record images since most output devices including monitors and printers are optimized for sRGB.
Nowadays, it is said that the color gamut that can be represented by the sRGB color space is narrow. In order to solve this problem, the following techniques, which are roughly classified into two types, are employed.
A first technique is to prevent generation of a clipping effect. A typical technique of this type is esRGB proposed by Hewlett-Packard and the like to extend the color gamut of sRGB.
For example, it is assumed that a luminance signal Y is created by:Y=0.3R+0.59G+0.11G  (1)
In this case, with respect to the luminance signal Y (Y=250), when increasing the chroma of a red hue, an R signal immediately reaches 255 and is not further increased no matter how hard it is tried to increase the chroma. Increasing the chroma by force produces a clipping effect where the R signal is clipped to 255. In the worse case, the hue may change, resulting in a color shift.
In contrast, esRGB has a luminance signal Y within the range of 0 to 255 (8 bits), whereas RGB signals are not limited to the range of 0 to 255. Specifically, esRGB extends the color gamut by allowing the R signal to be 256 or greater even when the luminance signal Y remains 250. In other words, esRGB extends the color gamut by allowing the individual RGB signals to be values other than 0 to 255.
The esRGB color space described above has just been proposed and has not been generally used yet.
A second technique employs a color space where the RGB tristimulus values are defined as values larger than those in sRGB. Such a color space may be, for example, NTSC (National Television System Committee), PAL/SECAM (Phase Alternate by Line/Sequential Couleur a Memoire), or Adobe RGB.
When an image recorded in a large color space, such as the above-described Adobe RGB, PAL/SECAM, or NTSC, is displayed on a regular sRGB monitor, the chroma is reduced to generate a less impressive image. In order to compensate for the reduction in the chroma, color space conversion must be performed. This requires additional processing and may cause conversion errors.
In order to avoid the necessity of color space conversion, the image must be displayed on an output device that is dedicated for each color space. Since Adobe RGB, PAL/SECAM, and NTSC represent extended color gamuts using the RGB signals within the range of 0 to 255, the accuracy with respect to the color gamut is more degraded than sRGB.
As described above, at present there is no color space that satisfies all factors. In such circumstances, as in high-end digital cameras, such as EOS-1D that has already been proposed by the assignee of the present invention, image recorders that can select a color space from among a plurality of color spaces are increasing in number.
When capturing an image of a subject in an extended color space by such a high-end digital camera, that is, when capturing an image of a subject with high chroma, NTSC or Adobe RGB is used to record the image. Otherwise, sRGB, which has high bit accuracy and which does not require subsequent color space conversion, is used. In this manner, the appropriate color space is selected and used in accordance with the application and/or the subject.
Similarly, one technique for adjusting the captured image to fit within the color gamut of the preset color space at the time the image is captured involves a method of adjusting image parameters. Specifically, for example, when capturing an image of a subject with high chroma, the image is captured with lower chroma and lower contrast (lower than the normal chroma and contrast). When capturing an image of a subject with low contrast and low chroma, the image is captured by increasing the contrast and chroma. In other words, the method adjusts the captured image to fit within the color gamut by changing image generating parameters, particularly tone and chroma parameters.
As in the case with the color space, this technique often requires additional retouching, after the image has been captured, for adjusting the contrast and chroma in accordance with the preferences of the person who has captured the image. It thus becomes necessary to appropriately select the contrast and chroma in accordance with the application and/or the subject.
As in the above-described method of adjusting the captured image to fit within the color gamut of the standard color space, a method is proposed for adjusting the color gamut of the captured image to optimally fit within the color reproduction range or color reproduction characteristics of an output device, such as a printer or a monitor. For example, when outputting an image of a subject with high chroma using a printer having a wide color reproduction range, the image remains unchanged and output as it is (with high chroma). In contrast, when outputting such an image using a printer having a narrow color reproduction range, the chroma and the contrast are suppressed and then the image is output. This technique is advantageous in a case in which the image is directly output without using a PC, e.g., in a case in which an image is captured by a camera and directly output from the camera to a specific printer.
Similarly, one technique for adjusting the captured image to fit within the color gamut of the preset color space at the time the image is captured is realized by a method involving changing the exposure. Specifically, for example, when an image of a subject with very high contrast is captured, high luminance portions and low luminance portions of the captured image are not transformed into gradations (tones). As a result, white elements in the high luminance portions and black elements in the low luminance portions are eliminated, causing color shifts. In order to capture an image whose high luminance portions fit within the color gamut, the exposure is reduced. In order to capture an image whose low luminance portions fit within the color gamut, the exposure is increased. This method is based on the fact that a color space has narrower color gamuts in high and low luminance portions and wider color gamuts in intermediate luminance portions. Such characteristics apply equally to a standard color space and to the individual color gamuts of devices and the like. This method requires the user to determine the exposure at the time the image is captured. The user must determine whether or not the captured image falls within the color space at the time the image is captured.
Using the above-described known technology, it is difficult to detect a color space that is most appropriate to a subject whose image is being captured. Therefore, it is very difficult to select the most appropriate color space.
In other words, with the above-described known technology, the user must use one color space and forget about the other wider color spaces, or the user must make efforts to perform color space conversion while accepting the accuracy degradation.
Similarly, when selecting the image generating parameters, it is difficult to select the most appropriate image generating parameters for a subject whose image is being captured.
Similarly, when selecting the most appropriate color space and image generating parameters for the color reproduction range and color reproduction characteristics of a specific output device, it is difficult to select the most appropriate image generating parameters for a subject whose image is being captured.
Similarly, when selecting the most appropriate exposure for the color reproduction range and color reproduction characteristics of a specific color space and/or a specific output device, it is difficult to select the most appropriate image capturing conditions for a subject whose image is being captured.